Questions tagged [indexed-variable]

Questions about indexed variables and related alternative methods to allow assigning indexed names to objects.

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6
votes
1answer
74 views

Undefined Indexed Variable

I searched already a lot about indexed Variables, and it tends to be the most applicable way to use tensor notation. But I am having a hard time to solve for undefined indexed variables: Here my ...
1
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1answer
73 views

Can I overload Part for specific heads (a la overloaded array subscripting)?

Mathematica/Wolfram Language newbie here. The behavior I am after is to be able to "overload" the subscripting of me-defined objects/Heads and thereby use them as if they were ...
2
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0answers
50 views

Referring to matrices with index

For exampe I have two matrices, with index 1 and 2 Mat[1] = ConstantArray[0, {2, 2}] Mat[2] = ConstantArray[0, {2, 2}] I wanna assign new value with ...
3
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2answers
120 views

How to use a list of indexes as a variables inside a Module?

This is a trivial function, just to show that Module does not recognize the list generated by Table. ...
1
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1answer
31 views

How does Mathematica evaluate sums where the upper limit of the index is not an integer?

I am interested in the way Mathematica evaluates sums where the upper limit of the index is not an integer - i.e., sums that are some sense mathematically meaningless. For example: ...
3
votes
2answers
104 views

Define implicit dummy index?

I've been looking for a solution to this question for a while. I was wondering if anyone is aware of a way of defining symbolic dummy indices to implement Einstein's convention. Let me explain with an ...
0
votes
1answer
61 views

Subscripts use to make expression readable [duplicate]

I am trying to write a function in mathematica which depends on different parameters. For example $$f(x_1, x_2, x_3, x_4, x_5)= 2x_1+\sin(x_2)+x_3*x_4-x_5^{-1}$$ (My actual function has 12 such ...
6
votes
2answers
99 views

How to create a nested list of variable depth with indexed elements

i want to obtain this definition of a nested list t = Table[C[i, j], {i, 3}, {j, 3}] with arbitrary depth, ...
3
votes
0answers
59 views

Parallelize import of many files

I'm trying to speed up the import of a large number of files, using ParallelTable to store them in a indexed variable, eqDump. The files are in the folder "Frames" and are named "Conf_Run.1000", "...
1
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1answer
35 views

Getting a stored variable from its value

Given a numerical value, is it possible to check whether that value is stored in (or defines) some variable in the notebook? Maybe if I could list all of the defined variables, then I could simply ...
6
votes
2answers
250 views

Defining a function of arguments with sub- and superscripts

I want to define a function that takes, several, let's say two arguments, both of which have several sub- and suberscripts. For example a function might have the signature $$ f(x_i,q_{j,k}) = \mathrm{...
1
vote
1answer
40 views

Manipulating variables indexed by a list

I have a variable g, indexed by a list: g[{1,1}]=11; g[{1,2}]=12; g[{2,1}]=21; g[{2,2}]=22; I would like to move things around as follows: ...
6
votes
1answer
642 views

Best way to write an equation with subscripts and superscripts

This post is not about a specific Mathematica problem but about to develop the best way to enter mathematical equations with subscripts and superscripts. I have a ...
2
votes
1answer
262 views

xAct, xTensor: How to avoid clash of indices?

Please refer to the picture below. In the first line, I define the angular momentum vector $\vec{L} = \vec{R} \times \vec{P}$ using the Levi-Civita tensor $\epsilon^{i}_{jk}$. The definition relies on ...
1
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2answers
119 views

Collecting terms from expression with indexed functions

Say I have an expansion of terms containing functions y[j,t] and its derivatives, indexed by j with the index beginning at 0 ...
0
votes
1answer
43 views

Getting named functions from an sequence of parameter values applied to a parametric function [closed]

The general question is how can I loop on names and indices? An example: I have pfun = ParametricNDSolveValue[{y'[t] == a y[t], y[0] == b}, y, {t, 0, 10}, {a,b}] ...
0
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2answers
193 views

Summation over an index and a set

I have an index CC and a set of values CM, and c is an element of CM and ...
2
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1answer
53 views

Loop with indexed function not defined for some index values [duplicate]

If anyone cares, this is supposed to be a very simple implementation of a Metropolis algorithm for a uniform distribution. I want to plot the variance of the sampled values as a function of an ...
8
votes
1answer
480 views

Einstein summation convention for symbolic vector calculus

I am trying to do some vector calculus in Mathematica in index notation form because it gives a clear result that can be compared to pen and paper calculations. Since there is no built in Einstein ...
0
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2answers
129 views

Sum over integer partition with variable function argument

Define $$\hat{X}(Y) = [X,Y] $$ I have known matrices $S_i$ and $V$. I am trying to use Mathematica to define a function which calculates $$ \sum_{\substack{n_1, \ldots, n_k>1\\ n_1+\ldots n_k = ...
1
vote
1answer
96 views

Create list of values with arbitrary index and the use it in a function

I have the following generating functions: $l_{2i-1}=l_{1}-(i-2)(w+s)$ with $i\geq 2$ and $l_{2i}=l_{2}-(i-1)(w+s)$ with $i\geq 2$, so the first one is for odd index and the second for even index. ...
4
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1answer
57 views

Taking a Part of a Part of a Variable

Say I have Part of a variable x. Part[x, 1] Then, I'll get an error message, as the ...
2
votes
2answers
49 views

Naming indexed output streams

I need to write into multiple files in a Do loop. Hence, I tried to open multiple streams: ...
0
votes
1answer
45 views

How to truncate equation with multivariables

I am trying to set certain terms zero. How to set terms with greater than 'a3' and 'an3' zero in the following equation: ...
2
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0answers
81 views

ParallelDo with doubly-indexed iterators

What is the reason that ParallelDo works with singly-indexed iterators, but not doubly- or more-indexed iterators? For example, the code ...
1
vote
0answers
55 views

Symbolic summation with variable bounds and variable number of indices

I wish to compute compute terms like Sum[f[t[j[1]],t[j[2]],...],{j[1],m},{j[2],n},...] for arbitrary positive integer n and any ...
0
votes
1answer
127 views

Multivariable functions symmetric in their arguments/representing indexed coefficients

Context: I have equations for indexed coefficients $C_{ij}$ which I have represented using 2-variable functions in Mathematica. e.g., ...
1
vote
1answer
126 views

subscript index notation select certain value [closed]

I have an indexed variable Subscript[p, s, m, t] established using the Notation method. There are various expressions where s, <...
1
vote
1answer
63 views

How do we input powers of indexed variables?

I have tried (a[i])^2 and alternatively (a^2)[i] Apparently both seem to work, but I was wondering what was the right way ...
0
votes
1answer
152 views

How do I get Mathematica to invert a matrix with subscripts involved? (Subscript usage)

I want Mathematica to invert a matrix in which all variables are symbolic including Subscript. Now I am guessing this has something to do with the method I'm ...
3
votes
2answers
573 views

Combining Multiple Lists

Let's say I have 4 lists: list1 = {a,b,c,d}; list2 = {e,f,g,h}; list3 = {i,j,k,l}; list4 = {m,n,o,p}; I can combine them using Join ...
29
votes
5answers
2k views

What are the requirements for a well behaved indexed variable? Subscript, ToExpression, Downvalue?

On writing this answer I needed to call a function, (NonlinearModelFit) with an unknown number of parameters. We have learned that we should not use ...
6
votes
1answer
2k views

When to use indexed variables

Still learning the fundamentals of the language I would like to ask you what advantages there might be in writing something like: a[1] = 2; a[2] = 4; a[3] = "x"; ...